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.Dd February 1, 2024 .Dt AIO_READ 2 .Os .Sh NAME .Nm aio_read , .Nm aio_read2 , .Nm aio_readv .Nd asynchronous read from a file (REALTIME) .Sh LIBRARY .Lb libc .Sh SYNOPSIS n aio.h .Ft int .Fn aio_read "struct aiocb *iocb" .Ft int .Fn aio_read2 "struct aiocb *iocb" "int flags" n sys/uio.h .Ft int .Fn aio_readv "struct aiocb *iocb" .Sh DESCRIPTION The .Fn aio_read , .Fn aio_read2 and .Fn aio_readv system calls allow the calling process to read from the descriptor .Fa iocb->aio_fildes . The syscalls return immediately after the read request has been enqueued to the descriptor; the read may or may not have completed at the time the call returns.
p For the .Fn aio_read and .Fn aio_readv calls, the read begins at the offset .Fa iocb->aio_offset .
p The .Fn aio_read call will read .Fa iocb->aio_nbytes into the buffer pointed to by .Fa iocb->aio_buf , whereas .Fn aio_readv reads the data into the .Fa iocb->aio_iovcnt buffers specified by the members of the .Fa iocb->aio_iov array. For .Fn aio_readv the .Fa iovec structure is defined in .Xr readv 2 .
p The .Fn aio_read2 call takes the .Fa flags argument. If .Fa flags is passed as zero, the call behaves identically to .Fn aio_read . The following flags can be specified by logical or: l -tag -width AIO_OP2_VECTORED t AIO_OP2_FOFFSET The read occurs at the file descriptor offset, which is advanced by the operation as done by the .Xr read 2 syscall. The .Fa iocb->aio_offset field is ignored. t AIO_OP2_VECTORED Similar to .Fn aio_readv , the read buffers are specified by the .Fa aiocb->aio_iov array. .El
p The .Fa iocb pointer may be subsequently used as an argument to .Fn aio_return and .Fn aio_error in order to determine return or error status for the enqueued operation while it is in progress.
p If the request could not be enqueued (generally due to invalid arguments), then the call returns without having enqueued the request.
p If the request is successfully enqueued, the value of .Fa iocb->aio_offset can be modified during the request as context, so this value must not be referenced after the request is enqueued.
p The .Fa iocb->aio_sigevent structure can be used to request notification of the operation's completion as described in .Xr aio 4 . .Sh RESTRICTIONS The Asynchronous I/O Control Block structure pointed to by .Fa iocb and the buffer that the .Fa iocb->aio_buf member of that structure references must remain valid until the operation has completed.
p The asynchronous I/O control buffer .Fa iocb should be zeroed before the system calls to avoid passing bogus context information to the kernel.
p Modifications of the Asynchronous I/O Control Block structure or the buffer contents are not allowed while the request is queued.
p If the file offset in .Fa iocb->aio_offset is past the offset maximum for .Fa iocb->aio_fildes , no I/O will occur. .Sh RETURN VALUES .Rv -std aio_read aio_read2 aio_readv .Sh DIAGNOSTICS None. .Sh ERRORS The .Fn aio_read , .Fn aio_read2 , and .Fn aio_readv system calls will fail if: l -tag -width Er t Bq Er EAGAIN The request was not queued because of system resource limitations. t Bq Er EFAULT Part of .Fa aio_iov points outside the process's allocated address space. t Bq Er EINVAL The asynchronous notification method in .Fa iocb->aio_sigevent.sigev_notify is invalid or not supported. t Bq Er EOPNOTSUPP Asynchronous read operations on the file descriptor .Fa iocb->aio_fildes are unsafe and unsafe asynchronous I/O operations are disabled. .El
p The following conditions may be synchronously detected when the .Fn aio_read or .Fn aio_readv system call is made, or asynchronously, at any time thereafter. If they are detected at call time, The calls return -1 and set .Va errno appropriately; otherwise the .Fn aio_return system call must be called, and will return -1, and .Fn aio_error must be called to determine the actual value that would have been returned in .Va errno . l -tag -width Er t Bq Er EBADF The .Fa iocb->aio_fildes argument is invalid. t Bq Er EINVAL The offset .Fa iocb->aio_offset is not valid, the priority specified by .Fa iocb->aio_reqprio is not a valid priority, or the number of bytes specified by .Fa iocb->aio_nbytes is not valid. t Bq Er EOVERFLOW The file is a regular file, .Fa iocb->aio_nbytes is greater than zero, the starting offset in .Fa iocb->aio_offset is before the end of the file, but is at or beyond the .Fa iocb->aio_fildes offset maximum. .El
p If the request is successfully enqueued, but subsequently cancelled or an error occurs, the value returned by the .Fn aio_return system call is per the .Xr read 2 system call, and the value returned by the .Fn aio_error system call is either one of the error returns from the .Xr read 2 system call, or one of: l -tag -width Er t Bq Er EBADF The .Fa iocb->aio_fildes argument is invalid for reading. t Bq Er ECANCELED The request was explicitly cancelled via a call to .Fn aio_cancel . t Bq Er EINVAL The offset .Fa iocb->aio_offset would be invalid. .El .Sh SEE ALSO .Xr aio_cancel 2 , .Xr aio_error 2 , .Xr aio_return 2 , .Xr aio_suspend 2 , .Xr aio_waitcomplete 2 , .Xr aio_write 2 , .Xr sigevent 3 , .Xr siginfo 3 , .Xr aio 4 .Sh STANDARDS The .Fn aio_read system call is expected to conform to the .St -p1003.1 standard. The .Fn aio_read2 and .Fn aio_readv system calls are FreeBSD extensions, and should not be used in portable code. .Sh HISTORY The .Fn aio_read system call first appeared in .Fx 3.0 . The .Fn aio_readv system call first appeared in .Fx 13.0 . The .Fn aio_read2 system call first appeared in .Fx 14.1 . .Sh AUTHORS This manual page was written by .An Terry Lambert Aq Mt terry@whistle.com . .Sh BUGS Invalid information in .Fa iocb->_aiocb_private may confuse the kernel.